A planetary or epicyclic gear system having its planet pinions organized in two opposing arrays on flexpins has the capacity to transfer a large amount of torque in a small package, that is to say, it possesses a high power density. See U.S. Pat. No. 7,297,086. The system of that patent utilizes spur gears. Its planet pinions rotate on flexpins that are cantilevered from two carrier walls and mesh with a sun gear and a ring gear. One of the carrier walls is connected to a torque reaction member that can either anchor the carrier against rotation or rotate and transfer torque from that wall directly into another drive line component farther down the system. Irrespective of whether the carrier is anchored against rotation or rotates, torque transfer from the torque reaction member to the flexpins on both carrier walls. The torque transfer path to the flexpins on the carrier wall to which the torque reaction member is connected will be inherently stiffer than the torque transfer path to the flexpins on the opposite carrier wall, because the torque transfer path to the opposite carrier wall also includes the webs that connect the two carrier walls and have an associated torsional spring rate that makes the combined spring rate for the flexpins on the opposite wall weaker. In other words, a design as depicted in U.S. Pat. No. 7,297,086 with spur gears, without consideration of how to achieve torque transfer equalization, will inherently transfer different values of torque through each planetary array, because there are different torsional spring rates in the two torque paths. The stiffer torque transfer path will transfer more torque and power than the weaker torque transfer path that includes the webs. This is a problem, because to take full advantage of flexpin technology to achieve maximum downsizing, a two-array planetary system must actually transfer the same nominal value of torque and power through each array and hence through each torque path. In other words, all planetary gears should transfer nominally equal torque.
PCT/US09133896 (WO 2009/102853) offers corrective measures that might be taken if flexpins are fitted with spur gearing as shown in the illustrations. Without correction, a typical value of torque transfer side to side in a spur gear system might be (for discussion purposes) on the order of 35% through the weaker torque transfer path and 65% through the stiffer torque transfer path. This means that in this example the stiffer torque transfer path would transfer 85% more torque than the weaker one. Ideally, all the gears and the bearings associated with them will benefit if both torque transfer paths can transfer equal values—that is, 50% of the total torque through each torque transfer path. PCT/US09/33896 discloses methods that are intended to achieve equal torque transfer in both transfer paths. These methods are described for a planetary gearing system shown equipped with spur gears and include using flexpins having different spring characteristics and locating the stiffer flexpins on the carrier side that is weaker; using less gear backlash in the gears on the weaker carrier side to force earlier gear contact, indexing the less stiff carrier side ahead of the stiffer side to force more load on it, and varying the stiffness of the carrier walls to achieve more equal global stiffness in both torque transfer paths.
Helical gears operate smoothly and often with less noise than spur gears. Moreover, the teeth on helical gears are longer and stronger than the teeth of spur gears of equivalent size. But helical gearing has not found favor in planetary systems that utilize flexpins because the planet pinions experience thrust loads at the meshes between the sun gear and planet pinions and at the meshes between the ring gear and planet pinions. Indeed, the thrust loads are in opposite directions and produce a couple that will tilt the planet pinions and disturb the mesh with the sun and ring gears unless counteracted. PCT/US2009/043007 (WO 2009/152306) discloses means for inhibiting the unwanted deflection that the couple would otherwise produce. But flexpins will otherwise maintain a good mesh between the planet pinions and the sun and ring gears, whether the gearing be spur or helical, and thus offer advantages over the end-to-end anchoring of planet pinion pins as in straddle-type carriers.